add find module for abseil that handles missing version

cpp/cmake_modules/FindabslALT.cmake

@@ -0,0 +1,48 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+if(abslAlt_FOUND)
+  return()
+endif()
+
+set(find_package_args)
+
+if(abslAlt_FIND_VERSION)
+  list(APPEND find_package_args ${abslAlt_FIND_VERSION})
+endif()
+if(abslAlt_FIND_QUIETLY)
+  list(APPEND find_package_args QUIET)
+endif()
+
+if(abslAlt_FIND_REQUIRED)
+  list(APPEND find_package_args REQUIRED)
+endif()
+
+find_package(absl ${find_package_args})
+
+if(NOT DEFINED absl_VERSION)
+  # Abseil does not define a version when build 'live at head'.
+  # As this is their recommended path we need to define a large version to pass version checks.
+  set(absl_VERSION 99999999_head)
+endif()
+
+set(abslAlt_VERSION ${absl_VERSION})
+
+find_package_handle_standard_args(
+  abslAlt
+  REQUIRED_VARS absl_FOUND
+  VERSION_VAR abslAlt_VERSION)